Add Env and SetEnv methods to command.Meta to retrieve the current
environment name inside any command.
Make sure all calls to Backend.State contain an environment name, and
make the package compile against the update backend package.
Since the data resource lifecycle contains no steps to deal with tainted
instances, we must make sure that they never get created.
Doing this out in the command layer is not the best, but this is currently
the only layer that has enough information to make this decision and so
this simple solution was preferred over a more disruptive refactoring,
under the assumption that this taint functionality eventually gets
reworked in terms of StateFilter anyway.
James Bardin